Structural organization and biological activity of molecular clones of the integrated genome of a BALB/c mouse sarcoma virus

Kirsten Ras* oncogene: significance of its discovery in human cancer research

The KRAS/ K-RAS oncogene is crucially involved in human cancer. The term "oncogene" -- i.e., a gene able to transform a normal cell into a tumor cell - was introduced in 1969, but the word was not used in the human carcinogenesis literature until much later. Transforming Kras and Hras oncogenes from the Kirsten and Harvey sarcoma viruses were not identified until the early 1980s due to the complicated structures of the viral genomes. Orthologs of these viral oncogenes were then found in transforming DNA fragments in human cancers in the form of mutated versions of the HRAS and KRAS proto-oncogenes. Thus, RAS genes were the first human oncogenes to be identified. Subsequent studies showed that mutated KRAS acted as an in vivo oncogenic driver, as indicated by studies of anti-EGFR therapy for metastatic colorectal cancers. This review addresses the historical background and experimental studies that led to the discovery of Kirsten Ras as an oncogene, the role of mutated KRAS in human carcinogenesis, and recent therapeutic studies of cancer cells with KRAS mutations.

Great migration: epigenetic reprogramming and germ cell-oocyte metamorphosis determine individual ovarian reserve

Emigration is defined as a synchronized movement of germ cells between the yolk sack and genital ridges. The miraculous migration of germ cells resembles the remigration of salmon traveling from one habitat to other. This migration of germ cells is indispensible for the development of new generations. It is not, however, clear why germ cells differentiate during migration but not at the place of origin. In order to escape harmful somatic signals which might disturb the proper establishment of germ cells forced germ cell migration may be necessary. Another reason may be to benefit from the opportunities of new habitats. Therefore, emigration may have powerful effects on the population dynamics of the immigrant germ cells. While some of these cells do reach their target, some others die or reach to wrong targets. Only germ cell precursors with genetically, and structurally powerful can reach their target. Likewise, epigenetic reprogramming in both migratory and post-migratory germ cells is essential for the establishment of totipotency. During this journey some germ cells may sacrifice themselves for the goodness of the others. The number and quality of germ cells reaching the genital ridge may vary depending on the problems encountered during migration. If the aim in germ cell specification is to provide an optimal ovarian reserve for the continuity of the generation, then this cascade of events cannot be only accomplished at the same level for every one but also are manifested by several outcomes. This is significant evidence supporting the possibility of unique individual ovarian reserve.

Development of transforming function during transduction of proto-ras into Harvey sarcoma virus

Oncogenic retroviruses are generated by transduction of the coding region of a protooncogene and acquire genetic changes during subsequent replication. Critical genetic events which occurred during and after transduction of rat proto-ras-1Ha into Harvey sarcoma virus were identified by evaluating the transforming activity of plausible synthetic progenitor proviruses encompassing the complete proto-ras genomic region with or without various 5' deletions. All progenitor proviruses induced phenotypic transformation of mouse NIH 3T3 cells, although with a 5- to 10-fold lower frequency than Harvey sarcoma provirus. Although no tumor formation was observed in vivo after inoculation in the absence of helper murine retrovirus, both wild-type and progenitor viruses inoculated in the presence of helper virus induced tumors in newborn BALB/c mice. No critical alterations of the p21ras coding region and no deletion of 5' genomic elements were detected in a progenitor virus encompassing the complete proto-ras genomic region that had been isolated from tumors. However, one progenitor virus that included all proto-ras exons induced tumors with a decreased latency. This virus contained a mutation in codon 12 (glycine to valine), which had apparently been selected during tumorigenesis in vivo. During the genesis of Harvey sarcoma virus, critical steps conferring transforming function are therefore transduction of coding proto-ras exons and enhancement of their transforming function by specific amino acid changes in p21ras.

Expression of multiple oncogenes in human esophageal carcinomas

To study the oncogenesis of human esophageal carcinoma, the expression of a variety of oncogenes was studied in 10 esophageal carcinoma cell lines and 16 pairs of tumor and nontumor tissues removed from patients with esophageal carcinoma. Northern blot analyses using 11 different oncogene probes revealed that 5 oncogenes, i.e. c-myc, c-H-ras, c-sis, c-raf, and c-fos, were expressed. Among them, a variant c-sis mRNA transcript of 2.7 kilobase (kb) was expressed in 7 of 10 cell lines and in 9 of 16 tumor tissues. Furthermore, an overexpression and an amplification of c-myc gene was observed in some cell lines. These results suggest that multiple oncogene expression may be required for the induction, maintenance, and progression of esophageal carcinoma. The expression of a 2.7-kb transcript, of c-sis and overexpression of c-myc gene may play some role in the carcinogenesis of esophageal carcinoma.

Protooncogene expression in rat liver by polychlorinated biphenyls (PCB)

1. The expression of 10 protooncogenes was studied in control rat liver and at various times after exposure to polychlorinated biphenyls (PCB), a known tumour promoter. 2. The expression of protooncogenes in liver is more pronounced in those rats treated with PCB beginning at weaning ('weanlings') than in adult rats. 3. The RNA levels of c-Ha-ras, c-raf, c-yes, c-erbA and c-erbB are elevated after PCB feeding. 4. Nuclear run-on transcription analysis revealed that the altered expression of the protooncogenes is transcriptionally regulated. 5. In one group the prompt rise of the protooncogene transcription rate is followed by a decline (c-Ha-ras, c-raf c-yes). In a second group a further increase in transcription at later feeding times (c-erbA, c-erbB) was observed. 6. A correlation between the altered expression of these protooncogenes and the action of PCB as a tumour promotor remains to be determined.

The specificity of the disease induced by defective murine retroviruses containing abl, fos, or Ha-ras is usually not determined by their LTR

The long terminal repeats (LTR) of the defective murine sarcoma viruses (MSV) containing v-abl, v-Ha-ras, or v-fos were exchanged for LTRs from other retroviruses having different tissue tropism. The new chimeric MSV were found to induce the same diseases as the parental viruses, indicating that sequences outside the LTR, most likely those of the oncogene, are responsible for the disease specificity of these defective MSV.

Altered oncogenes in UV-transformed C3H 10T1/2 mouse cells: identification of mutated H-ras allele(s)

Ultraviolet (UV) light will transform mammalian cells in culture to a phenotype which is characteristic of in vivo neoplasia. The UV-transformed C3H 10T1/2 mouse cell lines, TU-2 and TU-3, were analysed to determine the molecular mechanisms which may account for their phenotype, and to determine the types of mutations induced by UV light. DNA-transfection assays indicated that the transformed phenotype of TU-2 could not be transferred to non-transformed recipient cells. Therefore, studies were initiated to determine the mutagenic effects of UV light with respect to cellular oncogenes. Northern blot analysis indicated that five of the oncogenes analysed (erb-A, erb-B, mos, myb, and N-ras) were not expressed at detectable levels. The steady-state mRNA levels of fos, K-ras, abl, sis, and src oncogenes were similar in the C3H 10T1/2 and TU-2 cells. The mRNA levels of three oncogenes, raf, myc and H-ras, were 1.5-2.0-fold greater in the TU-2 cells compared to C3H 10T1/2. Southern blot analysis of HpaII restriction digested TU-2 DNA indicated that the H-ras oncogene has undergone methylation changes. More extensive analyses of the H-ras locus in TU-2 demonstrated a deletion of the 3' end of the gene, that may involve two separate mutated alleles. This type of damage is consistent with the lesions associated with sister chromatid exchange. While the H-ras locus in the other UV-transformed line, TU-3, showed methylation changes, there were no large genetic mutations detected by Southern blot analysis. These results suggest that UV-irradiation in vitro induces endogenous DNA damage that includes methylation changes and large genomic alterations. Further analysis will be necessary to determine the extent to which each may be involved in cell transformation.

Specific patterns of oncogene activation in transplacentally induced tumors

Transplacental exposure of rats to a single dose of the direct acting carcinogen methylnitrosourea (MNU) results in the induction of a variety of neoplasias of neuroectodermal, epithelial, and mesenchymal origin. Molecular analysis of the oncogenes present in these tumors revealed a striking degree of tissue specificity. neu oncogenes were found to be reproducibly activated in tumors derived from the peripheral nervous system (PNS), but not in those arising from the central nervous system (CNS). No ras oncogenes were found in either PNS- or CNS-derived tumors. However, Ha-ras oncogenes were detected in each of three mammary carcinomas and Ki-ras oncogenes were present in each of five kidney mesenchymal tumors. These results illustrate that phenotypic expression of activated oncogenes in vivo is not a random process and suggest that normal developmental programs may play an important role in modulating the activation of specific oncogenes by chemical carcinogens. PCR analysis revealed that each of the ras oncogenes detected in these transplacentally induced tumors became activated by the same G----A transition in the second base of codon 12. Since G----A transitions are the preferred mutations induced by MNU, it is likely that these ras oncogenes may have been directly targeted by MNU during embryonic development.

Recombinant BALB and Harvey sarcoma viruses with normal proto-ras-coding regions transform embryo cells in culture and cause tumors in mice

The ras genes of BALB and Harvey sarcoma viruses contain point mutations in codon 12 or codons 12 and 59, relative to proto-ras from normal animal and human cells. By in vitro recombination between cloned rat proto-ras and cloned BALB and Harvey sarcoma proviruses, we constructed recombinant proviruses with normal proto-ras-coding regions. These recombinant proviruses transformed mouse 3T3 cells upon transfection. However, when the transforming efficiencies of proviral DNAs were compared after transfection with helper provirus, recombinant proviruses were 2 to 30 times less efficient than the corresponding wild-type proviruses. Recombinant sarcoma viruses isolated from cells transformed by cloned proviral DNA contained the expected normal ras-coding region. They transformed rat embryo cells and induced erythroblastosis and sarcomas in newborn mice as efficiently as wild-type viruses did. We conclude that conversion of normal proto-ras genes to viral ras genes depends on truncation of normal proto-ras regulatory elements and substitution by retroviral (long terminal repeat) promoters and that the transforming function of long terminal repeat-ras genes is enhanced by point mutations.

Revertants of v-fos-transformed fibroblasts have mutations in cellular genes essential for transformation by other oncogenes

Morphologic revertants of FBJ murine sarcoma virus (v-fos)-transformed rat-1 fibroblasts were isolated using a novel selection procedure based on prolonged retention of rhodamine 123 within mitochondria of v-fos-transformed versus normal fibroblasts. Two classes of revertants were isolated: class I revertants have sustained mutations in cellular genes, and a class II revertant has a nonfunctional v-fos provirus. Somatic-cell hybridization studies suggested that the revertant phenotype was recessive to the transformed phenotype. Class I revertants were also resistant to retransformation by v-gag-fos-fox, v-Ha-ras, v-abl, and v-mos, but could be retransformed by the trk oncogene and polyoma virus middle T antigen. These results suggest that the class I revertants sustained mutations in one or more cellular genes essential for transformation by some, but not all, oncogenes. Our data suggest the existence of common biochemical pathways for transformation.

Biologic and molecular characterization of two newly isolated ras-containing murine leukemia viruses

A murine sarcoma virus (MSV) was recovered from an (NFS X NS.C58v-1) F1 mouse which developed splenic sarcoma and erythroleukemia 6 months after inoculation with a mink cell focus-inducing murine leukemia virus (MuLV) isolated from an NFS mouse infected with a wild mouse ecotropic MuLV. The MSV, designated NS.C58 MSV-1, induced foci of transformation in mouse and rat fibroblasts, and inoculation of mice of various strains 2 weeks of age or younger resulted in erythroleukemia and sarcomatous lesions in spleen, lymph node, and brain. The MSV provirus was molecularly cloned from a genomic library prepared from transformed non-producer rat cells. The 8.8-kilobase proviral DNA contained a 1.0-kilobase p21 ras coding segment which replaced most of the gp70-encoding portion of an MuLV, most likely the endogenous C58v-1 ecotropic virus. The ras oncogene is closely related to v-Ha-ras by hybridization, expression of p21 protein, and nucleotide sequence. It is nearly identical in sequence to v-bas, the only previously described transduced, activated mouse c-ras. At position 12 in the p21 coding region, arginine is substituted for the naturally occurring glycine present in c-ras. A second MSV isolate is described which is similar to NS.C58 MSV-1 except for a 100- to 200-base-pair deletion in the noncoding region of the ras-containing insert.

Biochemical and biological activities of N-ras proteins

Recombinant N-ras proteins, expressed and produced from synthetic genes cloned into E. coli, have been tested in vitro for GTPase and autophosphorylation activity. The genes corresponding to the assayed proteins were tested for their ability to transform NIH 3T3 cells. Mutations of glutamine to lysine at amino acid position 61 and glycine to valine at position 12 were both found to activate the ability of the N-ras gene to transform NIH 3T3 cells while significantly reducing the GTPase activity of the corresponding protein. N-ras proteins were also found to autophosphorylate in the presence of GTP when a threonine acceptor amino acid is provided at position 59.

Activating mutations of the c-Ha-ras protooncogene in chemically induced hepatomas of the male B6C3 F1 mouse

Activated c-Ha-ras protooncogenes have recently been identified in the DNA of some spontaneous hepatic tumors found in 2-year-old B6C3 F1 mice. Activation of c-Ha-ras has now been demonstrated in DNA from well-differentiated hepatomas initiated by a single dose of carcinogen given to male B6C3 F1 mice at 12 days of age. DNA from each of 25 hepatomas, induced by N-hydroxy-2-acetylaminofluorene, vinyl carbamate, or 1'-hydroxy-2',3'-dehydroestragole, containing transforming activity in the NIH 3T3 transfection assay. Southern analysis of NIH 3T3 cells transformed by DNA from 24 of these hepatomas revealed amplified and/or rear-ranged restriction fragments homologous to a Ha-ras probe. The other tumor contained an activated Ki-ras gene. Immunoprecipitation and NaDodSO4/PAGE analysis of p21 ras proteins in NIH 3T3 transformants derived from a majority of the hepatomas suggested that the activating mutations were localized in the 61st codon of the c-Ha-ras gene. Creation of a new Xba I restriction site by an AT----TA transversion at the second position of codon 61 was detected in DNA from primary tumors and NIH 3T3 cells transformed by DNA from 6 of 7 vinyl carbamate- and 5 of 10 1'-hydroxy-2',3'-dehydroestragole-induced hepatomas. Selective oligonucleotide hybridization demonstrated a CG----AT transversion at the first position of the 61st codon in NIH 3T3 transformants derived from 7 of 7 N-hydroxy-2-acetylaminofluorene-induced hepatomas. By the same criterion, an AT----GC transition at the second position of codon 61 was the activating mutation in 1 of 7 vinyl carbamate- and 5 of 10 1'-hydroxy-2',3'-dehydroestragole-induced tumors. Thus, c-Ha-ras activation is apparently an early event in B6C3 F1 mouse hepatocarcinogenesis that results directly from reaction of ultimate chemical carcinogens with this gene in vivo.

Expression of protooncogenes in murine osteosarcomas

The expression of 7 protooncogenes (c-sis, c-abl, c-mos, c-bas, c-Ki-ras, c-fos, c-myc) was examined in transplants and established cell lines from spontaneous and radiation-induced murine osteosarcomas. The transplant tumors were compared with different tissues, particularly skeletal tissue (sternum), and the osteosarcoma cell lines with fibroblast lines from the same mouse strains. C-sis was expressed above the level of controls in 2 osteosarcomas (TV, Os5). Three osteosarcomas showed over-expression of c-abl (TVK, DOS, Os5), c-bas (DOS, Os5 and V893) and c-fos (TVK, DOS, Os5), and 4 osteosarcomas showed over-expression of c-Ki-ras (TVK, DOS, Os5, Os16) and c-myc (TVK, DOS, TV, Os5). C-mos expression was not observed under the conditions used. One cell line (Os5) showed an altered transcript (1 kb transcript of c-fos). Apart from the relatively frequent increase in expression of the c-myc and c-ras-family, there was no indication that any particular protooncogene or combination of protooncogenes was associated with murine osteosarcomas.

Ras p21 proteins with high or low GTPase activity can efficiently transform NIH/3T3 cells

We sought to determine whether decreased in vitro GTPase activity is uniformly associated with ras p21 mutants possessing efficient transforming properties. Normal H-ras p21-[Gly12-Ala59] as well as an H-ras p21-[Gly12-Thr59] mutant exhibited in vitro GTPase activities at least fivefold higher than either H-ras p21-[Lys12-Ala59] or H-ras p21-[Arg12-Thr59] mutants. Microinjection of as much as 6 X 10(6) molecules/cell of bacterially expressed normal H-ras p21 induced no detectable alterations of NIH/3T3 cells. In contrast, inoculation of 4-5 X 10(5) molecules/cell of each p21 mutant induced morphologic alterations and stimulated DNA synthesis. Moreover, the transforming activity of each mutant expressed in a eukaryotic vector was similar and at least 100-fold greater than that of the normal H-ras gene. These findings establish that activation of efficient transforming properties by ras p21 proteins can occur by mechanisms not involving reduced in vitro GTPase activity.

Detection and identification of activated oncogenes in spontaneously occurring benign and malignant hepatocellular tumors of the B6C3F1 mouse

Species- and strain-specific spontaneously occurring tumors have been observed in rodents maintained under normal laboratory conditions. Elucidation of the molecular mechanisms associated with the development of these spontaneous tumors may provide a better understanding of tumor development associated with exposure to chemical carcinogens. In view of the high frequencies of oncogene activation shown in rodent tumors induced by known chemical carcinogens, we have investigated oncogene activation in spontaneous tumors of the B6C3F1 mouse and Fischer 344/N rat by DNA transfection techniques. A marked difference in the presence of activated oncogenes in spontaneous rat tumors versus spontaneous mouse liver tumors was observed in this study. All rat tumors tested failed to yield activated oncogenes (0/29), whereas 30% (3/10) of mouse hepatocellular adenomas and 77% (10/13) of hepatocellular carcinomas scored positive by DNA transfection. These transforming genes were identified as an activated Ha-ras gene in all the adenoma transfectants and in 8 of the 10 carcinoma transfectants. The two remaining hepatocellular carcinomas contained transforming genes that appear not to be members of the known ras gene family. The B6C3F1 mouse liver system might provide a very sensitive assay not only for assessing the potential of a chemical to activate a cellular proto-oncogene, but also for detecting various classes of proto-oncogenes that are susceptible to mutational activation.

Nucleotide sequence analysis of the BALB/c murine sarcoma virus transforming gene

We determined the nucleotide sequence of the v-H-ras-related oncogene of BALB/c murine sarcoma virus. This oncogene contains an open reading frame of 189 amino acids that initiates and terminates entirely within the mouse cell-derived ras sequence. The protein encoded by this open reading frame matches the sequence predicted for the T24 human bladder carcinoma oncogene product, p21, in all but two positions. The presence of a lysine residue in position 12 of BALB/c murine sarcoma virus p21 likely accounts for its oncogenic properties.

Gene expression during liver regeneration

The concentration of a group of messenger RNAs, some of which are expressed only (or mainly) in the liver and others of which are expressed in all tissues, was examined during liver regeneration. Most of the tissue-specific mRNAs did not change greatly in concentration or in transcription rate, but mRNAs such as actin and tubulin increased by as much as tenfold without an equivalent transcriptional increase. However, the mRNAs for "acute phase" proteins such as serum amyloid A and metallothionine did increase dramatically and increased transcription was easily detected. In addition to these findings, there was no increase in the rate of synthesis of the RNA constituents necessary to make ribosomes, pre-rRNA and mRNA for ribosomal proteins. Thus, the differentiated hepatocyte continues to function as a differentiated cell during the two or so replications necessary for regeneration, and many of the constituents necessary to increase cell mass may be supplied by increased preservation and utilization of transcribed RNAs.

Stimulation of 3T3 cells induces transcription of the c-fos proto-oncogene

Transcription of the c-fos proto-oncogene is greatly increased within minutes of administering purified growth factors to quiescent 3T3 cells. This stimulation is the most rapid transcriptional response to peptide growth factors yet described, and implies a role for c-fos in cell-cycle control. Transformation by c-fos may result from a temporal deregulation of this control.

Expression of normal and transforming H-ras genes in Escherichia coli and purification of their encoded p21 proteins

The H-ras gene of the BALB murine sarcoma virus (BALB-MSV) was placed under the transcriptional control of the tightly regulated PL promoter of bacteriophage lambda in the expression vectors pEV-vrf-1 and pRC23. Upon derepression of the PL promoter, large amounts (10-20% of total cellular protein) of the H-ras gene product p21 are synthesized in Escherichia coli. We constructed three H-ras gene expression vectors, designated pJCL-H5, pJCL-E30, and pJCL-33. pJCL-H5 directs the synthesis of p21, a fusion protein whose four amino-terminal residues are replaced by eight amino acids coded for by plasmid sequences. The 13 5' coding nucleotides of the BALB-MSV H-ras gene missing in pJCL-H5 were regenerated in pJCL-E30 by inserting a pair of complementary synthetic oligodeoxynucleotides. As a result, pJCL-E30 encodes a p21 protein, p21T, of sequence identical to that of the transforming p21 protein of BALB-MSV. pJCL-33 is a derivative of pJCL-E30 in which the 12th codon, AAA, a lysine codon, was replaced by GGA, a glycine codon. Thus, pJCL-33 directs the synthesis of a p21 protein, p21N, whose sequence corresponds to that of a normal cellular p21 protein. We report the purification of H-ras p21 proteins to apparent homogeneity by a method involving solubilization with chaotropic agents followed by reverse-phase high-performance liquid chromatography.

A transforming ras gene in tumorigenic guinea pig cell lines initiated by diverse chemical carcinogens

Fetal guinea pig cells were transformed by treatment with four different chemical carcinogens including nitroso compounds and polycyclic hydrocarbons. As a consequence of this treatment, oncogenes capable of transforming NIH/3T3 cells became activated in each of five independently established clonal guinea pig cell lines. Molecular characterization of representative NIH/3T3 transformants revealed that the same oncogene was present in each of the cell lines tested. Moreover, detection of this transforming gene paralleled the acquisition of tumorigenic properties by these neoplastic cells.

Induction of mammary carcinomas in rats by nitroso-methylurea involves malignant activation of H-ras-1 locus by single point mutations

Each of nine mammary carcinomas induced by a single injection of nitroso-methylurea into 50-day-old Buf/N female rats, contained a transforming H-ras-1 gene. Molecular characterization of one of the genes revealed that the twelfth codon was GAA instead of GGA of the normal allele, encoding glutamic acid in place of glycine. These results indicate that chemical carcinogenesis represents an adequate model to study the role of transforming ras genes in human neoplasia.

Transformation of Bloom's syndrome fibroblasts by DNA transfection

Nonmalignant diploid human fibroblast cells (GM3498B) derived from a skin biopsy of a patient with Bloom's syndrome have been transformed by transfection with DNA from a tumorigenic mouse cell line (Ha-8) carrying a single copy of the Harvey murine sarcoma virus (Ha-MuSV) genome. The transformed cell lines have an extended life-span, form colonies in agarose, and proliferate in nude mice--characteristics of neoplastic transformation. Like the parental cells, they also exhibit a high spontaneous level of sister chromatid exchanges. Finally, the transformed cells contain most, if not all, of the Ha-MuSV genome as well as the human rasH sequence. These experiments show that these diploid nonmalignant human cells can be used as recipients in transfection experiments for studying the genetic control of neoplastic transformation.

Increased expression of myc-related oncogene mRNA characterizes most BALB/c plasmacytomas induced by pristane or Abelson murine leukemia virus

RNA blots of poly(A)-containing RNA from normal livers and spleens and from a number of transplantable hematopoietic and lymphoid BALB/c tumors, including early and late generation plasmacytomas, were hybridized with probes for four onc genes. abl RNA was abundant only in those tumors producing Abelson virus, bas RNA was found in approximately equal amounts in normal tissues and plasmacytomas, and myb RNA was absent in normal liver and plasmacytomas. Normal liver and spleen RNA showed faint traces of myc hybridization, but myc RNA was increased in most plasmacytomas. In one plasmacytoma, TEPC 1165, a particularly abundant amount of myc RNA was found, principally as a 3.5-kilobase band. In the other plasmacytomas, bands of 2.4- or 1.8-kilobase myc RNA were found. Southern blots of DNA from tumors that contained 2.4-kilobase or larger myc RNA showed myc hybridization to an EcoRI fragment of about 21 kilobase pairs, similar to the myc band in normal DNA. EcoRI digests of DNA from two tumors that expressed myc RNA of 1.8 kilobases showed an additional smaller myc band, suggesting that the myc gene is rearranged in these plasmacytomas. The basis for increased myc gene transcription in plasmacytomas is not understood, but the evidence suggests that different mechanisms may be operating in different plasmacytomas. Apparently, neither myc gene amplification nor myc gene rearrangement is required for increased myc transcription.

Detection and cloning of human DNA sequences related to the mouse mammary tumor virus genome

Sequences related to the mouse mammary tumor virus (MMTV) genome have been detected in fragments of restricted human cellular DNA. These results were obtained by using recombinant DNA containing the MMTV proviral genome and lowering the stringency of blot-hybridization conditions. The MMTV genome also reacts with unique families of fragments in restricted cellular DNA from other mammalian species but not with salmon sperm DNA. A clone that reacted with labeled MMTV proviral DNA was selected from a human DNA library in Charon 4A. Under stringent conditions, a 3.7-kilobase MMTV-related EcoRI fragment of this clone hybridized with many of the same EcoRI restriction fragments of human cellular DNA detectable with MMTV proviral DNA under low-stringency conditions. Specific fragments of the human clone were shown to contain sequences related to the molecularly cloned gag, pol, and env regions of the MMTV genome.

Primate origin of the cell-derived sequences of simian sarcoma virus

We sought to identify the species of origin of the cell-derived (sis) sequences of simian sarcoma virus. A molecular clone comprised of sis DNA detected related nucleotide sequences at low copy numbers in normal cellular DNAs of species as diverse as humans and quail. The extent of hybridization and degree of base-pair matching with sis DNA were greatest with New World primate DNAs. The thermal denaturation curve midpoints of hybrids formed between sis and woolly monkey DNAs were indistinguishable from homologous sis DNA hybrids, establishing the woolly monkey (Lagothrix spp.) as the source of sis sequences. In comparative studies, sis was shown to be more conserved among mammalian species than unique-sequence woolly monkey cellular DNA. There was no detectable homology between sis and the cell-derived sequences of other fibroblast-transforming retroviruses. These findings indicate that sis is likely to be a unique onc gene among transforming retroviruses.

Generation of BALB-MuSV and Ha-MuSC by type C virus transduction of homologous transforming genes from different species

The nature of the cell-derived (bas) sequences of BALB-MuSV, a spontaneous mouse sarcoma virus isolate, was determined. Molecularly cloned bas sequences demonstrated no detectable homology with the onc genes of other mouse transforming viruses, but exhibited a high degree of sequence homology with the ras gene of the rat-derived Harvey murine sarcoma virus (Ha-MuSV) genome. The Ha-MuSV cell-derived sequence (ras) shared a colinear 750 bp region of homology with bas. Moreover, BALB-MuSV transformation was associated with the expression of high levels of a 21,000 dalton protein, immunologically related to the ras gene products, p21. Thus bas and ras represent retroviral transforming gene homologs that were independently transduced by mouse type C viruses from the genomes of different species.